Ion implanters are commonly used in the manufacture of semiconductor products for implanting ions into semiconductor substrates to change the conductivity of the material in such substrates or in pre-defined regions thereof. Ion implanters generally comprise an ion source for generating a beam of ions, a mass analyser for selecting a particular species of ions from the ion beam and means to direct the mass-selected ion beam through a vacuum chamber onto a target substrate supported on a substrate holder.
Most frequently, the ion beam cross-sectional area at the target substrate is less that the surface area of the substrate which necessitates scanning of the beam over the substrate using a one or two-dimensional scan so that the beam covers the whole surface of the substrate. Three scanning techniques are commonly employed in ion implantation, as follows: (i) electrostatic and/or magnetic deflection of the ion beam relative to a static substrate; (ii) mechanical scanning of the target substrate in two orthogonal directions relative to a static ion beam; and (iii) a hybrid technique involving magnetic or electrostatic deflection of the ion beam in one direction and mechanical scanning of the target substrate in another generally orthogonal direction.
An important objective in the fabrication of semiconductor wafers is to ensure that for any selective species of ions, the wafers are implanted with the correct ion dose and that the dose is uniform throughout and across the wafer or part of the wafer targeted to receive the implanted ions. At present, the semiconductor industry frequently demands a dose uniformity of 1% or better. Failure to achieve such standards is both time consuming and very costly due to the significantly high cost of the wafers themselves.
The dose delivered during an implant process is monitored by measuring beam current using an ion beam current detector (usually a Faraday cup) positioned ‘behind’ the wafer so that, as the beam and the wafer effect movement one relative to the other so that the beam is no longer obstructed by the wafer, the beam can fall on the Faraday detector. Where implantation of multiple wafers is concerned, this may be achieved by positioning the Faraday detector behind the movable (usually rotatably) wafer holder with one or more gaps/slits in the holder through which the beam can pass to the Faraday detector that is aligned with the general path of the ion beam. Such an arrangement is disclosed in U.S. Pat. No. 4,234,797. Where single wafer implantation occurs, the Faraday cup will normally be placed in a fixed position behind the wafer so that the beam impinges on the Faraday detector as the wafer is moved out of alignment with the ion beam after each single traverse or sweep of the ion beam across the wafer. Such an arrangement is described in British Patent Application No. GB0400485.9.
Upon transport to the wafer, ions in the ion beam may become neutralised and so lose their electric change. These neutrals will continue to travel in the ion beam with the ions and will also implant in the wafers. Existing beam current detectors can measure only the ionic current, i.e. they cannot detect any neutralised ions, and so will normally understate the true rate of delivery of desired species, including both ions and neutrals, in the ion beam. Beam ions are generally neutralised by collisions with residual gas molecules in the vacuum chamber and it is known that the proportion of ions which become neutralised increases with increasing residual gas pressure. Collisions may also result in the state of charge of beam ions being increased, e.g. from singly to doubly charged or reduced from doubly or singly charged, and both these effects can contribute to beam current measuring errors.
It has been recognised that there is a need to compensate for the understatement or overstatement of the Faraday detector. A true or corrected beam current would be a proper measure of the rate of delivery in the beam of particles (whether ions or neutrals) of the species to be implanted. With accurate monitoring of the true beam current, the implant process can be adjusted to ensure uniform dosing across the entire wafer.
An ion implanter is described in U.S. Pat. No. 6,297,510 that may be operated to determine the true beam current. The ion implanter includes a substrate holder that moves relative to the ion beam such that the ion beam is traced across a wafer along a series of scan lines forming a raster pattern. As the ion beam is scanned relative to the wafer, photoresist layers provided on the wafer outgas to cause a rise in the residual gas pressure within a vacuum chamber enclosing the substrate holder. Transits of the ion beam across the wafer are separated by periods where the ion beam is no longer incident on the wafer and so out gassing stops. The term “separating periods” is used herein to refer to the periods when the ion beam is not incident on the wafer between transits of the ion beam across the wafer. During these separating periods, a vacuum pump that continually pumps on the vacuum chamber can act to cause the pressure to fall towards the vacuum chamber's base pressure once more. This fall in pressure during the separating periods falls exponentially, with time t and can be represented as Pt wherePt=Poe−t/r  (1) (with Po being the pressure at t=0), and τ is a characteristic pump-down time constant for the vacuum chamber.
As noted above, the ionic current measured by the Faraday detector varies with the vacuum chamber pressure. Thus, the variation in the measured ionic current Im is a function of pressure P and can be expressed asIm=I0e−KP  (2) where Io is the true beam current (ions and neutrals) and K is a constant that can be determined for any particular implant recipe. Equations (1) and (2) can be combined to show that the ionic beam current Im during a separating period is given by                               I          m                =                              I            o                    ⁢                      e                          -                              (                                  K                  ⁢                                                                           ⁢                                      P                    o                                    ⁢                                      e                                          -                                              t                        τ                                                                                            )                                                                        (        3        )            that, taking natural logarithms, can be expressed asln Im=ln I0−KPoe−t/r  (4) Equation (4) is of the general form y=m×+c. Thus, measuring a set of ionic beam currents Im at known times during a separating period allows a graph to be plotted of lnIm against e−t/r. The true beam current Io can then be found from the intercept with the y axis (as c=lnI0 in this case). It will be appreciated that knowledge of the constant K is not required with this method (and in fact, K can be found from the gradient m=−KP0). However, the time constant τ must first be determined to allow the graph to be plotted. The time constant τ is determined by measuring two or more chamber pressures whilst the pressure drops in the vacuum chamber (either initially or during a separating period). Equation (1) can be rewritten asln Pc=ln Po−{fraction (t/τ)}  (5) that, like equation (4), is of the form y=m×+c. Fitting the two or more pressure measurements to this form allows the time constant τ to be determined as the gradient m=−τ−1.
Hence, after an initial determination of the pump-down time constant τ, a series of ionic beam currents may be measured at the end of a scan line during the separation period to determine the true beam current at that time. The true beam current will vary slowly over time, and this drift will be detected and the implantation process may be controlled so as to correct for the variation and ensure uniform dosing across the entire wafer. For example, if the true ion beam current is found to fall with time, the scan speed of the ion beam relative to the wafer may be slowed to achieve the same dosing.
A disadvantage of the method described in U.S. Pat. No. 6,297,510 is that it requires the pressure in the vacuum chamber to be measured in order to determine the pump-down time constant τ. This introduces a requirement for an additional detector capable of measuring the pressure within the vacuum chamber, and this is detrimental in terms of the complexity and expense of the ion implanter.